Extrusion apparatus



V. P. CARACCIOLO ETAL April s, 1958 EXTRUSION APPARATUS 2 Sheets-Sheet J Filed Feb. 4, 1954 INVENTORSl VincenPeterChmcwZo Robert Nord Peterson April 8, 1958 v. P. cARAccloLo Erm. 2,829,399

EXTRUSION APPARATUS 2 Sheets-Sheet 2 Filed Feb. 4, 1954 NW.. IDW

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INVENTORS Virage rd; PeterCaracczoIo Robert Nord Peterson BYl f y :ATTORNEY EXrRUsIoN mura-Raumsv Vincentyleter Caracciolo and RobertNord Peterson, Wilmington, Del., assignors to E. I.. du Pont de Ne-v mours and Company, Wilmington, Del., a' corporation of Delaware Application February 4, 1954, Serial No. 408,170

4 claims. (ci. ias- 12) This invention relates to a novel extrusion device and in particular this invention relates to an extrusiony device for transforming an aqueous` lslurry ofy a synthetic plastic into a dry extruded melt of that plastic.`

Many of the present day synthetic plastic materials are prepared in the form of granules which may then be transported in bulk form fromthe manufacturer yto the fabricator who may then use any of theY variety of methods for changing the bulk granular material into molded plastic articles. Some plastic materialsaresynthesized in the form of a slurry lin which case the liquid medium must be removed in some manner iftheplastic is to be prepared into a solid forml such `as granules or a sheet material or thev like. thetic plastics may be separated from such a liquid medium by means of filtration, yd'ecanting, drying, and i other means known to those skilled in the art. However,y there are materials such as the polyacetals and ,-in particular polyvinyl butyral which are conveniently prepared and easily handled in the form of an aqueous slurry, but when the liquid medium is removed theremaining plastic material becomes so tacky and sticky that an "ice 2 tions indicated by the letters A, B, Cpaud D. The section designated as- Ais known as the feed section.v Section B is known asthe compounding section. Section C is known as the extraction section. Section D is known as the extrusion section. The apparatusl consists essentially of a casing, 1, lined with barrelsZ, 3, and 4, into which there is Afitted a screw device, 5, which is rotatably d'riven inr the indicated direction by a suitable power source connected atv 6; An aqueous slurry of plastic particles vis continuously fed intor section A through feed hopper 7. The slurry isadvanced by the rotation of screw 5 through sections.A,;B, C, and Din that order. In section A thev slurry isV subjected to a wringing or compressing laction `whereby the major portion of the liquid medium (in the case of an aqueous slurry of polyvinyl butyral the liquid mediumis water) is removed by Some of the known synunmanageable mass of material results. .Accordingly 'i it would be highly desirable if such a slurryk could` be processed in a single operation to produce a` melted extrudate which might be in the form of aesheet or other desired shape.v

It is an object of this invention to provide an extrusion device into which may be fed a liquidslurry of polymeric materials and out of which there is attained a melted extrudate free of'k the liquid medium in the feed material. It is another object ofthis invention to provide a method andan apparatus for removing large quantities of liquid in a feed material by means of a wringing action followed by a vacuum extraction zone wherein most of theremaining liquid is removed. Other objects willy be apparent from the more detailed description of the invention given herein. In theattacheddrawings there is a schematic illustration of the apparatus of this invention.

Figure l is an overall cross sectional view of the extrusion device of this invention. Figure ,2 is an enlarged crossV sectional View ofV sections A and B of the device illustrated in Figure 1. Figure 3 is a graphic representation of the relationship between the helix angle of the screw and that of the grooves in thev cooperating barrel. Figure 4 is a partial cross sectional view of one embodiment of this invention in whichtwin screws are employed in the. extrusion device. Figure 5 is a cross sectional viewV taken at X-X inrFigure 4. Figure 6 isa cross sectional view taken at Y-Y in Figure 4.

In Figure l the'. entire device is divided into four secforcing the liquid to travelin a direction opposite to that of the plastic materiall and to be discharged through feed hopper 7. By the time'the plastic material has reached section B, it is a viscous melted material containingl a small amount of thefliquid-me'dium introduced with vthe feed into hopper 7. In sectionBv the root diameter of the screw'has been increased to such an extent that vthe channel through which the plastic is travelling is considerably reduced in size and accordingly the pressure on the plastic material is increased. lt is preferable that the root diameter of the screw is increased gradually from some point in the center of-section Ato the entrance of section B although such an increase inthe root`dia meter may take place iff` desired over something less than a pitch length of the screw. At the` entrance Vto sectionl C the root diameter of the' screw is reduced back to some decreased size which maybefthe same size as in sections A and. Dand accordingly pressure on the plastic material is. immediately reduced. In section C the barrels, such as 3 or 4, surrounding thescrew is cut away at the top so thatinspace 8 the plasticA ma# terial being carried between the screw threads'is not confined from the top. The space 8 is normally evacuated so that'the material leaving section B underv high rpressure and temperature will be subjected to a ash evaporation upon entering section-C and by reasonof this evaporation,

much of the remaining liquid inthe plastic material will be removed. Ifit is convenient or. desirable, the barrel which partially surrounds the screw in section C t may be heated" by steam. coils, electric coils, or the like. .Section D is merely an ordinary extrusion device wherein a smooth barrel 4v surrounds the screw 5 and plastic material is forced through the section and out a convenient die such as indicated at 9.

In Figure 2 there is shown an enlarged view of sections A and B of this-device., -For orientation purposes; feed hopper 7 and casing 1 are shownin'their proper relationship to the remainder of the device. Barrel 2 is provided witha series of 'grooves 10 which are connected as indicated at 11. to forma multitude of helical grooves over the entire surface of the barrel 2 and therefore over the entire length of section A. These grooves are irnportant in two respects. In the rst place, theslurry material entering through feed hopper 7 does notI adhere to the walls of barrel 2, probably because the water in the slurry acts as a lubricant, and hence the slurry material fills the spiral channels between screw threads such as indicated at 16 and turns with the rotationof screw 5 but is not advanced in the direction of intended travel because this material slips on the walls of barrel 2. By

incorporating the helical grooves 11 in barrel 2, the plastic material contained in the channelsbetween screw threads contacts itself rather than smooth metal surfaces. Grooves 11 become filled with plastic slurry, and the cohesion between. plastic particles being carried by the screw threads and the same material in grooves 11 is sufliciently great that the material in the screw thread channels tends to cling to the material in grooves 11. Because of this increased friction, the rotation of screw causes material inthe screwA threadchannel to advance in thedesired 'direction rather ythanto rotate with the `screw without any advance relative to the screw; The grooves toward the end of section A serve to increase the capacity of the extrusion in that a molten plastic material also moves through these grooves in the direction of all other plasticmaterial, that is, in the direction indicated by 17.`

The second-uscful `.purpose which is served by the grooves 11 isthat they oer a means by which water 4or other,V liquid medium used in the slurry entering feed hopper 7Vmay travelin the reverse directionl to that of the plastic material beingftransported by` screw 5.V As the pressure on vthe plastic material in the screw thread channels is increased, thewater or other slurry medium which islless viscous than the' plastic` material is forced out ofthe mass of moving plastic andl because resistance to ow of suchless viscous vmaterial decreases in the direction indicated by-arrow"18,- this liquid tends to flow in that direction. This liquid will fof course take the path of least resistance and because the path through grooves-1-1 istgenerally `shorter than thepath defined by screw threads 16 the slurry medium may ow Vthrough these grooves if they are not completely plugged with plastic material. Furthermore, there is in every extrusion device, such as indicated in Figure 2, at least some small clearance between the outside diameter of screw threads 16 and the inside diameter of barrel 2. The slurry medium may actually' short circuit the path of grooves 11 by flowing from one groove to an adjacent groove, and solon in a more or less` straight path to the opening of feed hopper 7. Whatever may be the actual course of the slurry medium inside theextruson device, there is` a continual discharge of slurry medium from feed hopper 7,- indicating a tlow'in the direction of 18 inside the extrusion device. If feed hopper 7 is `of water tight construction, the slurrytmedum may rise in thefeed hopper and be discharged as antov'erow or it may be continuously pumped out. However, feed hopper 7 may be constructed in the manner ofY afsieve and water may ow out through anyof a multitude of holes in feed hopperf7 which are suicientlyfsmall `to prohibit the loss of polymericiparticles but sutiieientlyA large to permit the passage ofwater; f f Since `the pressure applied to thel moving plastic mass increases as the mass. advances in the direction indicated by V17, the pressure will eventually reach a point where the` water is forced out of the plastic andbegins its travelin the direction of. 18 to be discharged. The exact point where this wringing action. will take place isV not known definitelyV and will vary with the nature of the feed material introduced at 7, and with thecharacteristics and design of theextrusion device. It is probable that :someiwringing action begins, immediatelyfupou introduction of the feed materialfrom 7 to the contines of the screw threads such as Vindicated at- I6' andl that the wringing action will continue through some length of section A as .the pressure on Vthe plastic material is increased. A i

In orderto accomplish the wringing'action at a definite point inthe apparatus and to accomplishthe wringing action in a more efficient manriefrfit is preferable, al= thoughnot necessary, to remove a portion of thev screw threads such as indcateda't 12inFigure2. Since there is no screwthread in thisr zone, thevplastiematerialjis forced into a compact annular surrounding theshaft of screw 5 and 'must be advanced byp'ressure being applied by the plastic material advancing into this zone. Consequently, pressure is sharply increased as the plastic enters this zone and the major portion of the slurry medium is squeezed out and forced in the direction of 18. Normally, one pitch length of the screw thread or a length equal to the diameter of the screw, whichever is greater, is sucient to accomplish the wringing action at this point, although a greater length will accomplish the same purpose. The removal of less than the above specied lengths of the screw thread may accomplish the desired result in certain embodiments of this invention, although these specified lengths are preferred for defining the screw thread interruption,

Plastic material leaving the zone indicated at 12is again picked up by the screw threads andisr advanced toward section B. In the space between the interruption ofthe screw threads and the entrance to section B the plastic material absorbs heat and acquires the nature'of a melted plastic material becoming sticky, and adhering to thev walls of barrel 2, and accordingly grooves 11 are no longer necessary after the` plastic acquires this adherent characteristic. Therefore, from the end of section A to the discharge of the plastic material from this device,.the barrel surrounding the screw 5 is smooth and does not contain anyigrooves such as indicated at 11.v A convenient and preferable method of increasing the pressure andtemperature of Athe plastic mass moving from the interrupted thread zone to the end of section `A is to have the root diameter of screw 5 increase gradually from the diameter employed in zone 12 to the diameter employed in section B. If desired, however, this increase in' root diameter may take place over a shorter length of-screw' 5, for example, over the distance vof a pitch length 'or less of screw 5.

Section B is short in length, usually no more than about two `[pitch lengths of screw 5. In this section the root diameter of screw 5 is at `its maximum value having been increased to the diameter shown at 13. f Such an increase in diameter olers a large resistance to the plastic material entering section B and accordingly the pressure on the plastic material and its temperatureare greatly increased as the material is forced through section B. Such a restriction as indicated at section B may serve more than one purpose since this restriction may act as a `metering device gauging the flow of plastic material through the entire extrusion device as well as increase the pressure and temperature of the plastic material justprior to its introductioninto section C shown rin Figure l. Since there'still remains in the plastic material a small amount of the slurry medium introduced at 7 ancl not removed by the wringing action in section A, the material may be subjected to a vacuum in section C to remove enough of the remaining slurry `medium to produce the desired con-V centration in the final product. This medium may be present as a mixture with the plastic and/0r dissolved in the plastic material. By increasing the pressure and temperature of the plastic material in section B and by sharply reducing the root diameter of screw 5 at 1S where the plastiematerial enters the vacuum zone of section C the 'evaporation of the slurry medium is greatly intensified by the sudden reduction in pressure at 15. In effect, there` is therefore obtained a ash evaporation effect which speeds the removal of the remaining slurry medium in section C. VIf it is desirable, barrel 3 may be supplied with a 'means for heating such that the plastic material carried through sections B and C may be heated to accomplish better evaporation. r

i {n Figure 3 there `is shown a graphic representation of the vrelation between the helical angle of screw 5 and that of grooves 11 in section A of this device. By defining vector 19 as the direction of the axisof screw '5., vector 20 is a direction at right angles to the axis vof yscrew y5. Vector 21 represents the tangent to the screw lthread at the point where the screw thread crossestlie axis in a projection view such as indicated by point 22 on Figure 2. Y Vector 23indicates,r nthegdvirectionk of, a tangent toa projection ofgrooves 11f.atthe intersection with axis 19, such as indicated at point 24. Angle 25vis herein defined as the helix; angleof screw andyangle 26 is defined as the helix angle of groovesll. The helix angle 25 may be of whatever value that is determined as being appropriate for.- a particular extrusion; process, although a convenient value is-an angle of about 418 which corresponds approximately to a screwfthread having its pitch equal to its outside diameter. It has been found that for efficiency andpracticali operation the angle 26 is preferably greater than angle 25 and may even approach 90 in which case groovesI 11 wouldA be longitudinal and the straight lines parallel to the axis 19. In general, angle 26, the helix angle of grooves 11, should be some value between angle 25, the helix angle of the screw, and 90 but in no case less than about 10, the preferred angle being from about 30 to about 60. The relationship of these helices angles may be stated in another way by reference to the pitch'length of these two helices thus the pitch length of theV helix-'described by the grooves 11 should be not less than the pitch length of the helix described by screw 5. A preferred `combination is to have the pitch length of grooves 11 be approximately twice the inside diameter ofbarrel 2 in combination with a pitch length of screw 5 approximately equal to the outside diameter of screw 5. L

The direction of the4 twist of grooves 11 is opposite to that of the twist of screw 5 so that if one is a right hand twist the other is a left hand twist.

The cross section of grooves as indicated atl() are conveniently formed in a rectangular shape although this is not a critical point. For a maximum efficiency the leading edge of each groove, that is the edge closer to feed hoppper 7, should be relatively sharp and deiine an angle not greater than 90. The` trailing edge of each groove, that is the edge closer to compounding section B, may be smoothly rounded, tapered, or sharp `with little diierence in efficiency occurring regardles of the shape of the trailing edge. It is generally preferable that the shape of the cross section of the grooves be shallow in a radial direction, that is, it is preferred that thewidth of a groove measured in an axial direction he no less than the depth of the groove measured in a radial direction. It is also preferable that the width of the groove be less than the width of the screw thread 16 at its outer diameter. The distance between grooves in' an axialdirection or in a direction perpendiculark to the' groove, whichever is more convenient, will depend uponthe nature of the material which is tobe processed by this -device. In general, a large number of shallow grooves not less than 0.01 inch deep, and preferably more than about 3/16 inch deep, and at least his inch wide will provide a more eicient wringing action and enhance the advancement of plastic material by the screw threads in section A of the extruder. It is convenient to have the distance between grooves be approximately equal to the width of the groove. As an example of the above preferred limitations, a 2 diameter screw having a pitch equal to its outer diameter might have a screw thread approximately thick at its outer diameter. The grooved barrel for such a screw might be provided with grooves approximately 3716 wide by l/s" deepand by spacing'l2 of such grooves equal distance around the interior of the barrel the distance between grooves would be approximately 1A". Grooves have been utilized successfully which have a depth of 0.01, although it is preferable that for screws having a diameter of 1" or greaterk that the groove depth be more than 0.01" and preferably notless than about 5&6. n,

In Figure 4 there is shown an embodiment of this invention in which two coacting screwsl are employed in the feed section, the compounding section, ,and the vacuum extraction section of the extrusion device. These three sections correspond to letters A, B, and C in Figure 1. Two screws, 5a and 5b are placed adjacent to one another and are identical in all manners throughout the first three sections of the extrusion device as mentioned above except that the screw threads 16a and 16b twist in an opposite direction with respect to each other. That is, screws threads 16a are a left-hand thread and `screw threads'16b are a right-hand thread. Other than this difference, the two screws are identical throughout the rst three sections of the device. It should be noted thatthe shafts, asindicatedV at 6a and 6b, are driven by suitable means and turn at the same speed in opposite directions to each other. Screw 5b terminates at the exit end of the vacuum extraction section as indicated generally at 27. In the feed section, the screws are sur!- rounded by grooved barrels 2a and 2b. In the compounding section and the vacuum extraction section, smooth barrels 3a' and 3b surround both screws 5a and 5b. The screws 5a' and 5b are so mounted that there is a minute clearance betweenl the outside diameters of the two re'` spective screws as indicated generally at 28. From point 27, where screw 5b terminates, the flow of plastic maj- .terial is forced into an extrusion section consisting of only a single screw lwhich is the extension of screw 5a,A this portion being exactly the same as section D shown in Figure 1.

Figure 5 indicates the general relationship of screws 5a and 5b, grooved barrels 2a and 2b, and the surrounding casing 1. It may be seen that there is a small clearance 28 between the outside diameter of each of the two screws. vFeed hopper 7 is shown in a position such that it may allow plastic material to be fed to the general area between screws 5a and 5b. The vacuum extraction section comprises a box-like extension 29 superimposed upon casing 1 and a cover 30 for extension 29. This extension serves the purpose of uncovering the space above the two sections 5a and 5b permitting the application of` a vacuum in this space so that water or other volatile material can be removed fromk the plastic material passing through this section of the extrusion device.

Figure 6 shows in greater detail the arrangement of the vacuum extraction section with an open space 31 extending substantially from the center line of screw 5a to the center line of screw 5b and upwards to cover 30. It may be seen therefore that as screws 5a and 5b rotate toward each other, plastic material moves from points 32 and 33respectively toward clearance 28 and thatr at the moment the plastic material leaves the constriction at either of points 32 or 33, it is subjected to a vacuum in space 31 causing theplastic material to give up its dissolved or adsorbed water or other volatile material;

The following examples are provided to illustrate and not to limit the scope of this invention.

Example 1 An extrusion apparatus lconsisting of a single screw and its barrel. with a feed hopper and an exit die was tested to determine the increase in throughput obtained by using grooves barrels as compared to smooth barrels. The screw diameter was 2" and the grooves in the barrel were l/g wide by V16 deep. The materials extruded were (l) an aqueous slurry of polyvinyl butyral containing approximately 70% solids, (2) granular polymethyl methacrylate, and (3) granular polyethylene. The grooved barrel which was utilized contained twenty evenly spaced longitudinally straight grooves. By reference to Figure 3 of the attached drawings the angle 26 of the barrel used in these runs was The helix angle of the screw is defined elsewhere herein and is the same as angle 2S of Figure 3 of the attached drawings. 'The results of these runs are set forth in Table Iv showing comparative'values of the extrusion of plastic materials ina smooth barrel and a barrel containing longitudinal grooves; In every case the grooved barrel produceda higher delivery rate.

estese@ Table I Amount 0i de- Screw (2ll outlivery (grains side diameter) per revolution of screw) f Feed material to Run Barrel extruder At l0 At ,Root Helix p. s. i. p. s. i. `diamangle, gauge gauge eter deextruextru- (inches) grecs sion sion prcspressure sure A..- 0. 5 10 Smooth. Aqueous slurry of 3. 9 3. 4

` pol vinyl butyral (70 0 solids). B-.- 0.5 .Groot/ed. -do 9.3 9.0 C-.- 0. 5 30 Smooth 3. 0 2. 2 D-- 0.5 Grooved. o 6.2 E... 1. 5 20 Smooth Granular poly- 8. 5 o. 1

methyl rncthaerylate. Y F-.- 1. 5 20 Grooved. dp 14.1 1l. 5 G 1. 5 20 Smooth Dry granular poly- 6. 5 6. 5

f ethylene. H. l. 5 20 Grooved. do 8.7 8. 6 I--- l. 5 30 Smooth. 5. 3 5. 2 K. 1. 5 30 Grooved. 7. 3 7. 3 L-.- 0. 5 10 Smooth 13. 9 13. 0 M 0. 5 10 Grooved. 16. 1 16. 1

` Example 2.-,-1n this 'example an aqueous slurry of polyvinyl butyral (approximately 70% solids) `was fed into a one-inch screw extruder maintained at about 150 C.,.and the delivery rate in grams per minute was measured a's a function of the angular speed of the screw in revolutions per minute. A comparison was made between results obtained with a smooth barrel and with a helically grooved barrel. The grooves were 0.01 deep by 0.0l5'wide. The grooves were cut along the barrel `in a helix having a direction of twist opposite to that of the screw and having a `pitch equal to one thread per inch. The screw had a pitch of 1" and a root diameter which increased from 5A at the feed entrance to 5% at the exit of the extruder over a length of 6". The results are listed in Table II showing the great improvement in throughput afforded by the use of a helically-grooved barrel.

The materials which may be processed by the eX- trusion device of this invention may be any of a variety of synthetic plastic material which are produced Iin admixture with a liquid material which is desirably removed before the plastic material is extruded or otherwise formed into its iinal shape. `For example, mixtures of water or organic liquids with polymers such as polyvinyl chloride, polyvinyl acetate, polystyrene, polyethylene, i Y

chlorosulfonated polyethylene', polyacrylates, polymethyl methacrylate, polyvinyl acetals, polyvinyl butyral,' synthetic rubber materials, and the like, may be introduced in admixture with such water or organic liquid and proccssed in the device ofthis invention so as to remove the water or `organic liquid and extrude a homogeneous blendedV melt of the desired 4plastic all in a single step.

Although the device of this invention has been'described in'terms of a single screw. and barrel, it is `not intended thatsuch a limitation be imposed on this apparatus or proeess.- Thefeatures of this linvention are equally applicable to multiple screw extruders.` One example of such is a'twin'scr'ew extruder, wherein two screws which are identical, except that one'hasa right-hand thread whilethe `other has a left-handfthread, are employed in a side-by-side relationship and are rotated toward each other in the nature of meshed gears or milling rolls. These multiple screws may be intermeshing or non-meshing with adjacent screws. A particularly good arrangement of a twin screwA extrusionldevice in this invention is the employmentof twin' screws as shown in Figures 4, 5 and 6. Y l

It is also intended that this device is not limited to the employment of screws and cooperating barrels having helical threads and grooves respectively, which have a constant pitch. In many embodiments of this invention it may be advantageous to employ a screw with a variable pitch in yone or more of the described `sections of the extrusion device. It may also be advantageous to employ, in the' feed section a barrel whose grooves have a variable pitch. Various combinations of variable and constant pitch threads and grooves will be apparent to those skilled in the art; for example, a screw with a variable pitch thread in a barrel with constant pitch grooves, a screw with a variable pitch in the feed section and compounding section and a constant pitch in the vacuum extraction section and extrusion section. These, and other combinations, arc intended to be included in the herein described invention.

The extrusion device of this invention is useful in the processing of many'of a variety yof plastic materials where it is desired to blend 4plasticizers, pigments, fillers, and other materials with a polymer in dispersion form and to produce an extrudate readily for molding into a desired shape. It will be appreciated that the exit of this device may be iitted with` any of a variety of molds, dies, calendering rolls and other known devices which are useful in transforming a molten plastic into sheets, rods, tubes, and articles of any shape whatsoever.

We claim: 1

1.` An extrusion apparatus comprising a barrel having a feed opening at one end and an opening at the other end from which the extrudate leaves the barrel, a helical screw rotatable with respect to' the barrel and adapted to advance material from the feedkend` to the extrudate end of the i barrel and a driving means for the rotatable screw, said apparatus comprising four sections in the following order: l) a feed sectionwherein the inside surface of the barrel is provided with amultiplicity of shallow, helical parallel groovesthe helix of whichfhas a direction of twist opposite Ifrom that of said helical'screw and has a pitch length greater than the pitch length of said helical screw, (2) a compounding section wherein the inside surface of said barrel is smooth and the annular cross sectional area between the barrel and `the root `of the screw is less than one-half of the corresponding cross sectional area in the feed section, (3) a vacuumlextraction section, throughout the length fof which, there is' provided access to the helical screw, rotating within a smooth, close-fitting barrel, and ameans by lwhich vacuum maybe applied to the space above the accessible portion of said screw, and (4) an extrusion section wherein said helical screw rotates within a smooth close-fitting barrel and is provided with an exit die through which plastic material may be extruded under pressure. v

2. The apparatus of claim l in which the thread ight of, said helical screw in said feed section is discontinuous for at least one pitch length of said helical screw, the-discontinuous portion being placed not closer to `the feed opening than one pitch 'length of said helical screw.

3. The apparatus. of claim lin which the pitch length of said helical screw is approximately equal to` the outer diameter lof said screw and the pitch length of said helical parallel grooves is approximately equal to twice the inside diameterof said b arrel, the grooves having a substantially rectangular` shape whereby the width is less than' the 9 thickness of a thread of said screw at its outermost diameter, and the depth of said groove is less than its width.

4. An extrusion apparatus of the character described comprising the combination of a casing, a pair of peripherally adjacent parallel interconnecting barrels in the casing, a rotatably mounted helical screw in each barrel, one screw having la right-hand thread while the other has a left-hand thread, a driving means operably connected to the two screws so as to cause the screws to rotate toward each other; said apparatus being composed of four connected sections in the following order (l) a feed section wherein the inside surface of each barrel is provided with a multiplicity of shallow helical parallel grooves, the helix of which has a direction of twist opposite to that of its cooperating helical screw and has a pitch length greater than the pitch length of said helical screw, (2) a compounding section wherein the inside surface of each of said barrels is smooth and wherein the root diameter of the helical screw is increased with respect to the root diameter of the helical screw in the feed section such that the free volume per unit length of screw in the compounding section is less than half of the free volume of a corresponding length in the feed section, (3) a vacuum extraction section, throughout the length of which, there is provided `access to the cooperating helical screws, rotating within smooth, close-fitting barrels, and a means by which a vacuum may he applied to the space above the accessible portion of said cooperating helical screws, and (4) an extrusion section wherein a single helical screw rotates within a single close tting smooth barrel and is provided with an exit die through which plastic material may be extruded under pressure.

References Cited in the le of this patent UNXTED STATES PATENTS 1,353,917 Lambert Sept. 28, 1920 1,904,884 Royle Apr. 1S, 1933 2,646,378 Haux July 7, 1936 2,370,469 Johnson et al Feb. 27, 1945 2,547,000 Gray Apr. 3, 1951 2,547,151 Braeseke Apr. 3, 1951 2,549,997 Zies et al Apr. 24, 1951 2,615,199 Fuller Oct. 28, 1952 2,744,287 Parshall et al May 8, 1956 

